1. Field of the Invention
The present invention relates to an impregnated soft, flat sealing gasket, particularly for use as a cylinder head gasket or an exhaust flange gasket in internal-combustion engines, and a manufacturing method therefor. Such gaskets are composed of a non-woven fiber mat which may be metal-reinforced and which is impregnated with an addition cross-linkable silicone resin that is cross-linked in its final state.
2. Discussion of the Prior Art
Modern soft gaskets for use as cylinder head gaskets or exhaust flange gaskets for internal-combustion engines, are generally composed of non-woven fiber mats comprised of organic fibers, inorganic fibers, synthetic fibers or mixtures thereof and are impregnated with liquid, cross-linkable impregnating agents. The fiber mats are impregnated primarily so as to increase their strength, improve their media resistance and increase their sealing efficiency. The non-woven fiber mats may be further strengthened or reinforced with metal inserts imbedded in the interior. The impregnating agent is subsequently cross-linked in the finished gasket, preferably by the use of heat.
Preferred prior art impregnating agents are primarily solvent-free, liquid polybutadienes, polyacrylates or systems including isocyanates or epoxides, such as those disclosed in U.S. Pat. No. 3,970,322. These impregnating agents, which may include peroxide or sulfur cross-linking agents, thermally cross-link in the finished gasket without emitting volatile substances which might bloat the soft material.
In their final, cross-linked state, these known impregnating agents are more or less hard and brittle, however, so that the impregnated gaskets do not have sufficient plasticity and are not usable for their intended function. For that reason, the impregnating agents are generally only partially cross-linked so that the gaskets retain their shape, but are still plastically or elastically deformable and able to conform to the uneven surfaces to be sealed when installed. The final cross-linking of the impregnating agent then takes place after installation due to the operating heat of the engine.
Partially cross-linked impregnating agents, however, still contain relatively large numbers of reactive groups which, when stored over long periods of time, continue to cross-link in an uncontrolled manner due to aging and environmental influences and thus cause the gaskets to harden. After long periods of storage, such hardened gaskets can no longer be used and therefore, special measures must be taken to protect partially cross-linked gaskets to be stored against further cross-linking during storage. For that reason, the search continues for suitable, substantially solvent-free impregnating agents which have an improved storage life.
Silicone resins have recently become known which are addition cross-linkable in their liquid, uncross-linked state. Polymethylvinyl silicones and silicone acrylates are examples of silicone resins, which, with the addition of platinum salt catalysts, thermally cross-link into elastically deformable or storable products. Manufacturers of such systems, however, caution the user in their product information that the ability of the formulated, cross-linkable system to cross-link sufficiently requires that the system remain free of amines, sulfur compounds and sulfur emitting substances so that the catalyst will not be poisoned.
According to German Offenlegungsschrift (published non-examined application) No. 3,245,664, silicone resins which addition cross-link under the influence of a catalyst have been used as impregnating agents for cylinder head gaskets. For gasket applications, the fiber mats themselves must be free of amines, sulfur compounds or sulfur donors in order to prevent poisoning of the catalyst. Since fiber mats generally contain natural fibers, which may be organic or inorganic, and vulcanizable latex binders, which contain the usual rubber additives, fiber mats that are free of catalyst toxins can be manufactured only at great expense. Additionally, the impregnating agent baths are mixtures of the liquid silicone resin starting composition and a platinum salt catalyst and can be used only for a limited period of time, e.g., for 16 hours as disclosed in German Offenlegungsschrift No. 3,245,664. However, baths with limited potlives cannot be used to economically mass produce cylinder head gaskets.